A typical overshot assembly comprises a tubular body in which a pair of lifting dogs is pivotally mounted. The lifting dogs pivot about a transverse axis on an axle which attaches the lifting dogs to the body. A first end of the lifting dogs is adapted for catching a spearhead point therebetween. A spring is disposed between the second opposite ends of the lifting dogs for biasing the first ends toward each other to a capture position where a spearhead point can be caught. The bias of the spring holds the first ends of the lifting dogs together about a caught spearhead point.
In order to release a captured spearhead point an operator pushes the second ends of the lifting dogs together against the bias of the spring thereby spreading the first ends of the lifting dogs and releasing the spearhead point. For safety and efficiency reasons it is critical that the lifting dogs do not accidentally or prematurely release a caught spearhead point. For example when retrieving an inner tube assembly from a hole using an overshot, serious injury or death can occur to an operator if the core barrel becomes accidentally released from the overshot while the overshot is being suspended near the top of a drill rig. Similarly, very expensive equipment can be lost downhole if it becomes detached from an overshot. There is therefore a need for an overshot that addresses these problems.